1. Field of the Invention
The present invention relates to a tolan compound which is useful as a liquid crystal material and to a liquid crystal composition containing this tolan compound.
2. Discussion of the Background
A liquid crystal display device has been widely utilized for a watch, an electronic calculator as well as a word processor, a television set, etc. Of these liquid crystal display devices, particularly frequently used is a liquid crystal display device which utilized optical anisotropy and dielectric anisotropy of a liquid crystal material.
A wide liquid crystal temperature range, a low viscosity to ensure a rapid electro-optical response rate, a suitable double refractive index (.DELTA.n) to provide a wide visual scope and a high contrast, a large dielectric anisotropy (.DELTA..epsilon.) to ensure a low driving voltage, and the characteristics to be chemically and optically stable are given as the characteristics required for a liquid crystal material to be used for liquid crystal display devices.
Liquid crystal compositions presently used are usually prepared by mixing a compound having a liquid crystal phase near room temperatures and a compound having a liquid crystal phase at temperatures higher than room temperatures. In order for a liquid crystal display device to be used outdoor the liquid crystal must be stable in the temperature range of -40.degree. C. to 90.degree. C. In addition, because of the temperature dependency of optical anisotropy and dielectric anisotropy, that is, because of occurrence of sudden change near the N-I point (nematic anisotropy phase transfer temperature), it is necessary to use a liquid crystal material with a high N-I point.
Furthermore, to ensure a wide visual angle and a high contrast the retardation of liquid crystal phase, .DELTA.n.multidot.d, wherein .DELTA.n is the double refractive index of the liquid crystal material and d is the thickness of liquid crystal layer, must be optimized. However, because the thickness of liquid crystal layer (d) is limited to a certain range and a high response rate is required for liquid crystal display devices actually used, the thickness of liquid crystal layer (d) tends to be thin. Because of this reason, a liquid crystal materials with a large double refractive index (.DELTA.n) is required.
A driving voltage is dependent on a threshold voltage Vth, whereas the threshold voltage Vth is inversely proportional to the square root of dielectric anisotropy (.DELTA..epsilon.). Accordingly, the threshold voltage Vth can be kept low when a liquid crystal material with a positive value of dielectric anisotropy (.DELTA..epsilon.) is used.
Although various liquid crystal compounds have been developed and put on use up to the present time, these is no single liquid crystal compound satisfying all the characteristics mentioned above. Therefore, in practice, several kinds of liquid crystal compounds having different characteristics or non-liquid crystal compounds are mixed and used. Such mixtures, however, are not necessarily satisfactory.
4-Alkyl-4'-alkoxytolan (German Patent No. 2226376) and 4-alkyl-4'-fuluorotolan (Japanese Patent Application Laid-Open No. 260031/1986) are examples of known tolan compounds which are used mixed for the above-mentioned purpose. The mixtures of these tolan compounds, however, are not necessarily satisfactory.
As described above, in spite of various studies in the past on liquid crystal compositions no material which sufficiently satisfies all above characteristics has been found. Each compound used for liquid crystal materials has merits and demerits in its characteristics. In addition, the degree of the above required characteristics is different depending on the liquid crystal display device. The development of a novel liquid crystal compound and a liquid crystal additive which can provide characteristics depending on the intended object of a liquid crystal display device has been desired. Accordingly, an object of the present invention is to provide a novel tolan compound which, if added to a liquid crystal composition, can raise the N-I point and increase the double refractive index (.DELTA.n) of the liquid crystal composition. Another object of the present invention is to provide such a liquid crystal composition comprising this novel tolan compound.